The present disclosure relates generally to well drilling operations and, more particularly, to well drilling operations involving magnetic ranging using a rotating magnetic source.
To access certain hydrocarbons in the earth, two or more wells or boreholes may be drilled with a certain spatial relationship with respect to one another; specifically, one borehole may be drilled such that it has a specific location relative to a previously drilled borehole. For example, heavy oil may be too viscous in its natural state to be produced from a conventional well, and, thus, an arrangement of cooperative wells and well features may be utilized to produce such oil. Indeed, to produce heavy oil, a variety of techniques may be employed, including, for example, Steam Assisted Gravity Drainage (SAGD), Cross Well Steam Assisted Gravity Drainage (X-SAGD), or Toe to Heel Air Injection (THAI). All such techniques may benefit by determining a borehole assembly (BHA) location relative to an existing well.
SAGD may generally involve two parallel wells separated by an approximately constant vertical separation distance (e.g., 4 to 6 m) and an approximately constant transverse horizontal separation distance (e.g., within 1 m) over a horizontal distance of roughly 500 m to 1500 m. The upper well in a SAGD well pair may be known as an “injector well.” The injector well may inject superheated steam into a heavy oil zone formation, creating a steam chamber to heat the heavy oil contained therewithin. The lower well in a SAGD well pair may be known as a “producer well.” When the heated heavy oil becomes less viscous, gravity may pull the oil into the producer well below, from which the oil may be extracted.
Conventional measurement while drilling (MWD) survey data may not provide sufficient accuracy to maintain a consistent separation distance between the injector well and the producer well. Indeed, the direction of a horizontal well may be measured and controlled to approximately +/−3°, and the inclination may be measured and controlled to approximately +/−1°, using conventional MWD sensors and good directional steering practices. However, such relatively small angles may produce large errors in the position of a long horizontal well. For example, a horizontal well with a 1000 meter length having a 3° drift may have a 52 meter lateral error at the toe of the well. If the same horizontal well has a 1° drift in inclination, the well may also have a 17 meter vertical error.
To drill one well, such as an injector well, with a certain spatial relationship with respect to an existing well, such as a producer well, conventional magnetic ranging techniques may be employed. However, determining a position of a BHA in the injector well relative to a magnetometer in the producer well may involve many time-consuming measurements using conventional magnetic ranging. For example, measurements may be taken at several distinct locations within one or the other well, and the magnetometer may be moved forward or backward a specified distance before taking each measurement. Additionally, such conventional techniques may necessitate that the two wells be essentially parallel, and may not provide a relative location until the BHA has drilled a distance at least equal to the inter-well separation distance.